Self-healing concrete is used to fill up the cracks after hardening, which is developed in structures with the help of bacterial reactions in the concrete.
Self-healing concrete is the process of self-healing up of cracks or self-filling of damages with the help of bacterial reaction in the concrete after hardening.
The use of technology has taken the construction standards to a new high level in modern days. To attain a very good, sustainable, and economic concrete construction, different procedures, methods, and materials are used.
In repetitive dry and wet cycles, it can be observed that small cracks that occur in a structure of width in the range of 0.05mm to 0.1mm get wholly sealed, but the need for other remedial work is required when the cracks are of greater width occurs.
Self-healing concrete is one of the alternative processes for cracks, and before and after the crack, this common problem of cracking in the building has many remedies.
For self-healing of cracks, the bacteria used are acid-producing, and these types of bacteria can be viable for over 200 years under dry conditions and can be in dormant cells. Thus, in the crack healing process, these bacteria act as a catalyst.
Types of Bacteria Used in Concrete:
In the self-healing concrete, there are various types of bacteria used, such as;
As a result of the biological reaction of non-reacted limestone and a calcium-based nutrient self-healing concrete is formed, and calcium nutrients called ‘Calcium Lactate’ special type of bacteria’s are known as’ ‘Bacillus’. These products are added to the wet concrete when the mixing is done while preparation of concrete. For around 200 years, these bacteria can be in the dormant stage.
The water seeps in the cracks when the cracks appear in the concrete, and spores of the bacteria germinate and start feeding on the calcium lactate consuming oxygen. Then, to insoluble limestone, the soluble calcium lactate is converted.
Then the insoluble limestone starts to harden, and then without any external aid filling, the crack automatically begins. It also helps prevent corrosion of steel due to damages and improves the durability of steel-reinforced concrete.
Preparation of Self Healing Concrete:
There are two ways for the preparation of self-healing concrete as given below;
Direct Application Method
Direct Application Method:
In this method, when the mixing of concrete is done, bacterial spores and calcium lactate are added to concrete directly. The average properties of concrete do not change with the use of this bacteria and calcium lactate.
To climatic changes, the bacteria are exposed, and they germinate and feed on calcium lactate and produce limestone when water comes in contact with this bacteria, and thus sealing the cracks is done.
In this method, concrete is prepared by the bacteria and its food such as calcium lactate which are placed inside treated clay pellets, and for making self-healing concrete, about 6% of the clay pellets are added.
The sealing of cracks is done when concrete structures are made with bacterial concrete and when the damage occurs in the construction, and clay pellets are broken. The bacteria germinate, eat down the calcium lactate, and produce limestone, which hardens, and cracks are sealing.
This method is costlier than direct application but most commonly used.
Chemical Process of Self Healing Concrete:
Calcium hydroxide is produced with the help of bacteria which acts as a catalyst when the water comes in contact with the un-hydrated calcium in the concrete. With atmospheric carbon dioxide, this calcium hydroxide reacts, which forms water and limestone.
In the concrete, the limestone then hardens itself and seals the cracks.
Test & Result of Self Healing Concrete:
On regular concrete and self-healing concrete, standard tests were conducted and for 7 and 28 days test conducted were compressive and flexural strength tests on a concrete cube.
Compressive Strength Test Result for 28 and 7 Days:
Advantages of Self Healing Concrete:
There are the following advantages such as;
Without any external aide, self-repairing of cracks.
When compared to the usual concrete significant increase in compressive strength and flexural strength.
It helps in the reduction in permeability of concrete and resistance towards attacks of freeze-thaw.
It improves the durability of steel-reinforced concrete and reduces the corrosion of steel due to crack formation.
To human life, Bacillus bacteria are harmless.
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